We got a news release today from the state announcing that State Route 840, the southern bypass of Nashville which took so long to complete, has been accepted by the federal government as part of the Interstate system. Starting this week, the state will begin replacing the black-and-white, rectangular S.R.840 logos with shield-shaped, red-and-blue I-840 logos.
I was talking about this with a co-worker. He noted that 840 doesn't actually go from one state to another. I explained to him that it was just a bypass being added to the Interstate highway system, which led to a discussion of how the Interstates are numbered. Tennessee had long anticipated S.R. 840 becoming an Interstate, and chose the number 840 because it knew that would eventually be the interstate number. That way, the highway wouldn't have to change numbers (which would have been confusing). The highway was built to Interstate standards with the idea of someday making it a part of the Interstate system.
Here's how the system works: regular Interstate highways have two-digit numbers. Odd-numbered highways, such as I-65, run north and south. Even-numbered highways, such as I-24 or I-40, run east and west.
For north-south routes, the numbers start in the west and get higher as you go eastward. For east-west routes, the numbers start in the south and get higher as you go north.
Three-digit numbers indicate either a bypass/outer loop/beltway (such as I-440 or the newly-christened I-840) or a spur line (to borrow a railroad term). In either case, the last two digits refer to the main Interstate route with which the bypass or spur line is connected. Both I-440 and I-840 are connected to I-40. The first digit is even if we're talking about a bypass or loop, odd if we're talking about a spur line.
The spur line from I-81 running into Kingsport used to be I-181. However, when I-26 was extended all the way to the Tri-Cities, it connected to I-181, and so the spur line was no longer a spur line. It was re-branded in 2007 and became just the last few miles of I-26.
The most unusual exception to this rule involves routes known as Interstate H-1, Interstate H-2, Interstate H-3 and Interstate H-201. They are, administratively, part of the Interstate Highway System, even though they aren't -- and will never be -- useful for driving from state to state. That's because they're all located on the island of Oahu, in Hawaii.
Alaska and Puerto Rico both receive funds from the Interstate Highway program, but unlike Hawaii they just use local signage and numbering conventions rather than adopting the distinctive red-and-blue Interstate signs. Technically, there are Alaska highways known to the federal government as Interstates A-1, A-2, A-3 and A-4, but the locals don't know them by those numbers, and parts of them are not built to the normal Interstate standards.